23 Special Case Bearings

Navigation

NAU 102

Lesson 23

Special Case Bearings

We can determine a running fix mathematically as well as graphically.

Using:

•Rules of Thumb

•Bowditch Table 18


Rules of Thumb

Double the Angle on the Bow

Bow and Beam

7/10 Rule(2 points/4 points Rule)

7/8 Rule(30-60-90 Rule)


Double the Angle on the Bow

When the angular distance of an object on the bow is doubled, the

distance run between the bearings equals the distance from the object at

the second bearing.


Example

You are on course 193° T at 20 knots. At 0800 you sight a light bearing 157° T. At 0836, the same light bears 121° T. What is the distance off the light at the second

bearing?


1st angle on bow =

193° - 157° = 36°

You are on course 193° T at 20 knots. At 0800 you sight a light bearing 157° T. At 0836, the same light bears

121° T. What is the distance off the light at the second bearing?

2nd angle on bow = 193° - 121° = 72°

Distance Run = = 12 nm 36 min

60 min/hrX 20 nm/hr

At 0836, the light bears 121° T at 12 nm.


Bow and Beam

When the 1st bearing is 45° on the bow and the 2nd is 90°, the distance run between the bearings equals the

distance from the object when abeam.


Example

You are on course 193° T at 20 knots. At 0840 you sight a light bearing 238° T. At 0855, the same light bears 283° T. What is the distance off the light

when abeam?


1st angle on bow =

238° - 193° = 45° 2nd angle on bow = 283° - 193° = 90°

Distance Run = = 5 nm 15 min

60 min/hrX 20 nm/hr

At 0855, the light is abeam at 5 nm.

You are on course 193° T at 20 knots. At 0840 you sight a light bearing 238° T. At 0855, the same light bears

283° T. What is the distance off the light when abeam?


7/10 Rule

When the 1st bearing is 22.5° (2 points) on the bow and the 2nd is 45° (4 points)

0.7 X distance run = the distance off when abeam.

0.7 X time run + time of 2nd bearing = ETA abeam.


Example

You are on course 090° T at 10 knots. At 1020 you sight a light 2 points to port. At 1100, the same light bears

045° T. At what distance and time will you be abeam the light?


1st angle on bow =

2 points = 22.5° 2nd angle on bow = 090° - 045° = 45°

Distance Run = = 6.7 nm 40 min

60 min/hrX 10 nm/hr

Distance off abeam = 6.7 nm X 0.7 = 4.7 nm

You are on course 090° T at 10 knots. At 1020 you sight a light 2 points to port. At 1100, the same light bears 045° T.

At what distance and time will you be abeam the light?

Time abeam = 40 min X 0.7 + 1100 = 1128


7/8 Rule

When the 1st bearing is 30°on the bow and the 2nd is 60°

0.875 X distance run = the distance off when abeam.

0.5 X time run + time of 2nd bearing = ETA abeam.


Example

You are on course 295° T at 15 knots. At 1215 you sight a light bearing 325° T. At 1300, the same light bears 355° T. At what distance and time will you

be abeam the light?


1st angle on bow =

325° - 295° = 30° 2nd angle on bow = 355° - 295° = 60°

Distance Run = = 11.25 nm 45 min

60 min/hrX 15 nm/hr

Distance off abeam = 11.25 nm X 0.875 = 9.8 nm

Time abeam = 45 min X 0.5 + 1300 = 1322

You are on course 295° T at 15 knots. At 1215 you sight a light bearing 325° T. At 1300, the same light bears 355° T. At what

distance and time will you be abeam the light?


The problems can also be solved using Bowditch Table 18.

Running Fix


Example

You are on course 180° T at 22 knots. At 1400 you sight a light bearing 200° T. At 1430, the same light bears 218° T. What is your distance off at 1430? At what distance will you be abeam the

light?


1st angle on bow

200° - 180° = 20° 2nd angle on bow 218° - 180° = 38°

You are on course 180° T at 22 knots. At 1400 you sight a light bearing 200° T. At 1430, the same light bears 218° T. What is

your distance off at 1430? At what distance will you be abeam the light?


Distance Run = = 11 nm 30 min60 min/hr

X 22 nm/hr

Distance at 2nd bearing = 11 nm X 1.11 = 12.21 nm

You are on course 180° T at 22 knots. At 1400 you sight a light bearing 200° T. At 1430, the same light bears 218° T. What is

your distance off at 1430? At what distance will you be abeam the light?

Distance off abeam = 11 nm X 0.68 = 7.5 nm

Introduction to Navigation

Questions?

23 Special Case Bearings

Documents

Transcript of 23 Special Case Bearings